A polarizing effect can be generated in glasses containing silver, copper or copper-cadmium crystals. These crystals can be precipitated in a boroaluminosilicate glasses having compositions containing suitable amounts of an indicated metal and a halogen other than fluorine.
The polarizing effect is generated in these crystal-containing glasses by stretching the glass and then exposing its surface to a reducing atmosphere, typically a hydrogen containing atmosphere. The glass is placed under stress at a temperature above the glass annealing temperature. This elongates the glass, and thereby elongates and orients the crystals. The shear stress that acts on the particles is proportional to the viscosity of the glass and the draw speed during elongation. The restoring force that opposes the deformation by the shear force is inversely proportional to the particle radius. Hence, the optimum conditions for producing a desired degree of particle elongation and a resulting polarizing effect at a given wavelength involves a complex balance of a number of properties of the glass and the redrawing process. Once the glass has been elongated, the elongated glass article is then exposed to a reducing atmosphere at a temperature above 120° C., but not over 25° C. above the glass annealing point. This develops a surface layer in which at least a portion of metal halide crystals present in the glass are reduced to elemental silver or copper.
The use of silver halide as a polarizer material capitalizes on two properties of the silver halide that are (1) the liquid particle is very deformable, and (2) it is easier to make larger and controlled particles sizes. The disadvantages of using silver halide are (1) that one cannot make polarizers that operate at wavelengths shorter than red (approximately 650 nm) because of the refractive index of the silver halide and (2) that the process required a hydrogen reduction step. It is possible to stretch silver particles in glass as described in by E. H. Land in U.S. Pat. No. 2,319,816 and later by S. D. Stookey and R. J. Araujo in Applied Optics, Vol. 7, No. 5 (1968), pages 777-779. However, the problems encountered are the control of particle size and distribution, especially for visible polarizer application where the aspect ratio of the particle is smalls, typically 1.5-2 to 1.
The production of polarizing glass, as is described in the patent references provided below, broadly involves the following four steps:                1. Melting a glass batch containing a source of silver, copper or copper-cadmium and a halogen other than fluorine, and forming a body from a melt;        2. Heat treating the glass body at a temperature above the glass strain point to generate halide crystals having a size in the range of 500-2000 Angstroms ({acute over (Å)});        3. Stressing the crystal-containing glass body at a temperature above the glass annealing point to elongate the body and thereby elongate and orient the crystals; and        4. Exposing the elongated body to a reducing atmosphere at a temperature above 250° C. to develop a reduced surface layer on the body that contains metal particles with an aspect ration of at least 2:1.        
Glass polarizers, the material compositions and the methods for making the glasses and articles made from the glasses have been described in numerous United States patents. Products and compositions are described in U.S. Pat. Nos. 6,563,639, 6,466,297, 6,775,062, 5,729,381, 5,627,114, 5,625,427, 5,517,356, 5,430,573, 4,125,404 and 2,319,816, and in U.S. Patent Application Publication No. 2005/0128588. Methods for making polarizing glass compositions and or compositions containing silver, and/or articles made from polarizing or silver-containing glasses have been described in U.S. Pat. Nos. 6,536,236, 6,298,691, 4,479,819, 4,304,584, 4,282,022, 4,125,405, 4,188,214, 4,057,408, 4,017,316, and 3,653,863. Glass articles that are polarizing at infrared wavelengths have been described in U.S. Pat. Nos. 5,430,573, 5,332,819, 5,300,465, 5,281,562, 5,275,979, 5,045,509, 4,792,535, and 4,479,819; and in non-U.S. patents or patent application publications JP 5-208844 and EP 0 719 741. The Japanese patent publication describes a copper-based polarizing glass instead of a silver-based polarizing glass.
While there have been considerable efforts in the art to improve polarizing glasses and the methods used to make them, there is still considerable need for further improvement. In particular, it would be advantageous to have a glass and a method for making the glass that does not require the use of a reducing atmosphere step. While it possible to stretch silver (Ag) particles, there are very considerable problems with regard to controlling particle size and distribution. These difficulties are particularly pronounced regarding visible light polarizers where the aspect ratio is small, typically 1.5-2 to 1. Accordingly, it is the object of the present invention to provide a polarizing glass composition that does not require a reducing atmosphere step and a method for making such glass. In particular, it is an object of the present invention to provide a polarizing glass composition utilizing silver and an additional selected noble metal, wherein the additional noble metal is used to nucleate atomic silver to silver metal particles without the use of a reducing atmosphere step, and a method for making such glass.